An electrically heated catalyst or electric heating catalyst (hereinafter referred to as “EHC” in some cases), in which the catalyst is heated by a heat generating element that generates the heat by applying the electricity, has been developed as an exhaust gas purification catalyst provided for an exhaust gas passage of an internal combustion engine.
The following technique is known. That is, a catalyst is supported by a mat in a case for accommodating the catalyst. An inner tube, which is an insulating member for electric insulation, is interposed by the mat to provide a double tube at the outside of the catalyst, and the inner tube is allowed to protrude toward the upstream side and the downstream side from the mat (see, for example, Patent Document 1).
Further, the following technique is known. That is, a case, which accommodates a catalyst, is allowed to have a tapered shape on the upstream side and the downstream side from the catalyst, and a double tube is formed, in which the upstream side is closed and the downstream side is open at a portion of the tapered shape (see, for example, Patent Document 2).
According to the double tube as described above, it is possible to secure the insulation performance to some extent even when condensed water and particulate matter (hereinafter referred to as “PM”) adhere. However, it is feared that PM may adhere depending on the direction in which the exhaust gas flows. Therefore, it is feared that the insulation performance may be deteriorated. For this reason, it is feared that the electricity may flow through the case.